This invention relates to welding processes and apparatus that can be described generically as consumable electrode arc welding processes and apparatus. Specific examples of welding techniques that can be employed in practising this invention are GMA (gas metal arc) welding, submerged arc welding and flux cored electrode welding.
The instant invention is particularly useful in the welding of pipelines, but its applications are by no means limited thereto. However, by way of example, the instant invention will be described herein in detail as it applies to pipeline welding, but this is not to be taken as limiting.
In the welding of pipelines several different techniques using automatic welding equipment have been tried. Thus, the CRC process is a GMA welding process requiring special joint preparation including the formation of a chamfer on the inside edges of the pipe sections to be joined. In this process the root pass is accomplished internally by welding torches that are located inside the pipe sections.
In the "ESSO" (trade mark) process the pipe sections to be joined are butted together and a saw blade that immediately precedes the welding torch cuts a 0.060" gap at the butt joint. A 0.040" consumable electrode is inserted into this gap. This electrode must be kept directly in the centre of the gap if consistent results are to be obtained. In practice this centering has been found to be so difficult to maintain that the "ESSO" process essentially is not used commercially.
Although it is not used extensively in pipeline welding, the TIG (tungsten inert gas) welding process is used extensively in areospace and nuclear welding, another application to which the instant invention may be applied. In the TIG process an arc is struck between a tungsten electrode and the base material being welded. The base material melts and a filler rod is fed into the pool of molten metal and itself melts. The TIG process is a relatively slow process (2" to 12" of weld per minute).
The other technique commonly employed in welding pipelines is conventional welding using flux coated stick electrodes. Those skilled in the art are well aware that there are many problems inherent in stick electrode welding.
During the GMA welding operation both large and small drops of metal melt off the welding electrode at random times. The occurrence of a large drop may interfere with the arc stability and thus a small defect may be created. When the arc is unstable it often causes one side of the article (base material) being welded to become hotter than the other, and it is generally accepted that poor heat distribution results in poor strength characteristic of the completed weldment.
An important factor in welding is the penetration that is obtained. Penetration is a function of heat input, and if the arc is not stable, it is necessary to slow down the welding operation to achieve the required penetration. Because of this welders commonly slow down the travel speed and consequently put into the base material more heat than is required. Putting more heat than is required into the base material is undesirable, however, since it degrades the metallurgical physical properties of the base material.